1. Field of the Invention
The present invention relates to a semiconductor device, which is used as a fast switching device or a power device, and method of manufacturing the same.
2. Description of the Related Art
The voltage of power supply used in a CPU for a computer or the like has been lowered gradually. In accordance with this tendency, synchronous buck converter has been employed heavily. In a synchronous commutating power source, an elevation of the operating frequency increases the voltage variation rate dV/dt to the potential at a node between a high-side element and a low-side element. Consequently, even though a switching element at the low-side element is set at the off state, the off state can not be retained and is turned into the on state due to dV/dt of the drain voltage because of the gate-drain capacity in the switching element. This is known as the self turn-on phenomenon.
To prevent occurrences of the self turn-on phenomenon, a reduction in gate resistance in the switching element is important as well as a reduction in ratio (hereinafter, Cgd/Cgs) of the gate-drain capacity (hereinafter, Cgd) to the gate-source capacity (hereinafter, Cgs). In a power source for electronic instruments such as a note PC, a ratio of the output voltage to the input voltage is small, and the duration of the low-side element being kept on is longer than the duration of the high-side element being kept on. Therefore, a low on-resistance is also important.
To achieve such the property, structures of semiconductor devices as various switching elements have been considered. For example, the invention disclosed in Patent Document (JP 2004/327598 A) teaches two gate electrodes in a trench, and a source electrode formed as sandwiched therebetween via an insulator film. In this invention, the overlapped area between gate-drain is minimized to reduce the gate-drain capacity. If the overlapped area between the gate electrode and an n−-type epitaxial layer is small, the effect of depleting from the gate electrode toward the n−-type epitaxial layer can not be exerted. Accordingly, it is not configured to reduce the on resistance sufficiently.